A polymer electrolyte membrane fuel cell (PEMFC) is an apparatus for generating electricity through an electrochemical reaction of hydrogen and oxygen, and does not require adjustment of an electrolyte because it achieves a high efficiency as compared with another type of fuel cell, achieves a high current density and a high output density, achieves a short startup time, and uses a solid electrolyte. Further, because a reaction product of the polymer electrolyte membrane fuel cell is pure water, studies on the polymer electrolyte membrane fuel cell are being actively made in automobile industries as an environment-friendly power source.
The fuel cell may obtain high electrical energy by stacking several cells having a voltage to achieve high power. The stack of the individual components is referred to as a fuel cell stack, and generally hundreds of fuel cell stacks or more are stacked to drive a fuel cell vehicle.
The cells in the fuel cell stack include a membrane-electrode assembly that functions as a passage for hydrogen ions for an electrochemical reaction, a separator configured to move a reaction gas and electrons, a gas diffusion layer configured to uniformly decompose the reaction gas, and a gasket for separating hydrogen as the reaction gas, air, and cooling water when the components are stacked to prevent the hydrogen, the air, and the cooling water from being leaked to the outside. The membrane-electrode assembly is classified into an electrolyte membrane and electrodes, and the electrolyte membrane used for the membrane-electrode assembly is mainly famed of a solid polymer, and a thin membrane is used to lower ion conductivity that greatly influences the performance of the fuel cell.
The high polymer electrolyte fuel cell is generally operated at an operation temperature of about −30° C. to 80° C. due to the characteristics of the polymer membrane. The polymer membrane requires conductivity for a high performance. It is the content of water that influences the conductivity most greatly. The excessive amount of water existing in the cells badly influences the performance and durability during an operation of the fuel cell. This is called flooding. Because the excessive amount of water hampers arrival of a gas to the electrodes in view of performance, it greatly increases material transfer resistance, which causes a decrease of performance and fluctuation of cell voltages. Further, in view of durability, the excessive water, in particular, the water existing in the anode may influence corrosion of carbon of the cathode electrode as well as corrosion of carbon of the anode electrode, and because it may greatly influence the performance of the fuel cell vehicle, a measure for preventing it is necessary.
When a fuel cell stack is manufactured and operated, the unevenness of supply of a fluid may be caused according to the cell design and deviation of quality. FIG. 1 is a view illustrating a fuel cell stack 1 that supplies air from the lower side to the upper side. FIG. 2 is a view illustrating an air lifted separator.
Referring to FIG. 1, the fuel cell stack 1 may include a plurality of unit cells 2 that are stacked, a closed end plate 3a and an opened end plate 3b coupled to the outermost unit cells 2. The opened end plate 3b may include an air inlet 4IN and an air outlet 4OT through which air is introduced and discharged, a hydrogen inlet 5IN and a hydrogen outlet 5OT through which hydrogen is introduced and discharged, and a cooling water inlet 6IN and a cooling water outlet 6OT through which cooling water is introduced and discharged.
Referring to FIG. 2, an air lifted separator may include an air inlet manifold 7IN and an air outlet manifold 7OT communicating with the air inlet 4IN and the air outlet 4OT, and a hydrogen inlet manifold 8IN and a hydrogen outlet manifold 8OT communicating with the hydrogen inlet 5IN and the hydrogen outlet 5OT. Further, the separator may include a cooling water inlet manifold 9IN and a cooling water outlet manifold 9OT communicating with the cooling water inlet 6IN and the cooling water outlet 6OT.
The air lifted separator and the fuel cell stack 1 may improve non-humidification characteristics and low-humidification characteristics, but because water cannot be easily discharged as compared with horizontal supply of water, flooding may occur as the water cannot be easily discharged (see the air outlet manifold of FIG. 2). In particular, because a larger amount of water flowing toward the outlet of the fuel cell stack 1 is collected, a differential pressure of the gas in the passage of the separator for the outlet of the stack further increases. The increased differential pressure of the gas decreases the gas supply speed, and the flooding is accelerated.
FIG. 3 is a graph depicting a characteristic result of distribution of a plurality of cell voltages measured in a condition of 30° in which flooding occurs by using the fuel cell stack 1 to which the air lifted separator as an example. In FIG. 3, as the number of the cells decreases, the cells are disposed closer to the outlet of the stack (the opened end plate).
FIG. 4 is a view schematically illustrating a side sectional surface of the fuel cell stack 1 of FIG. 1. In FIG. 4, the thickness of the arrow facing the upper side indicates a flow rate of a reaction gas flowing from the lower side to the upper side, and the hatched part indicates an amount of residual water of the condensate existing in the outlet manifold.
As in the embodiments of FIGS. 3 and 4, as the condensate discharged to the gas outlet manifolds of the separators of the cells is connected in the outlet of the stack, a larger amount of condensate is distributed at the outlet of the stack in the outlet manifold as illustrated in FIG. 4, and thus the difference between the flow rates of the reaction gas passing through the channels may become larger as it go towards the outlet of the stack. Accordingly, as in the graph of FIG. 3, the performance of the cells disposed at the outlet of the stack may be lowered.